The present invention relates to centrifugal pumps, and more particularly to centrifugal pumps used for transporting slurries and other abrasive-containing fluids.
Vast tonnages of solid-liquid mixtures, known as slurries, are pumped every year. The largest quantities are pumped in the dredging industry, which continually requires maintaining navigation in harbors and rivers, altering coastlines and winning material for landfill and construction purposes. As a single dredge may be required to maintain a throughput of 7000 tons of slurry per hour or more, very large centrifugal pumps are used.
The manufacture of fertilizer is another process involving massive slurry-transport operations. Phosphate is recovered by huge draglines in open-pit mining operations which is then slurried and pumped to wash plants through pipelines with a typical length of about 10 kilometers. Each year some 34 million tons of matrix are transported in this manner. The phosphate industry employs centrifugal pumps that are generally smaller than those used in large dredges, but impeller diameters up to 1.4 m are common, and drive capacity is often in excess of 1000 kW. The transport distance is typically longer than for dredging applications, and hence a series of pumping stations is often used.
Many other types of open-pit mining operations use slurry transport, and the number of such applications is increasing as it becomes clear that, for many short-haul and medium-haul applications, slurry transport is more cost-effective than transport by truck or conveyor belt.
Partially processed material from mining and metallurgical operations and other industries is often already in slurry form, facilitating pipe transport. Much of this is carried out using relatively small lines.
Since the purpose of a slurry pipeline is to transport solids, the higher the concentration of solids in the pipeline the more efficient and less costly it is to pump per unit of solids being transported. While it is easier to pump fine solids, grinding such solids to a small size is costly, thus large sized solids are commonly pumped. Larger sized solids and higher concentrations, however, result in higher wear inside the pump and particularly wear to the impeller vanes and shrouds.
The major erosive mechanisms resulting in pump wear are sliding abrasion and particle impact. The sliding-abrasion mode of wear typically involves a bed of contact-load particles bearing against a surface and moving tangentially to the surface. In pipelines, the stress normal to the surface is caused by gravity. The submerged weight of the particles not suspended by the fluid must be carried by intergranular contacts. The analysis of the motion of these contact-load (bed-load) solids in pipes has been developed over many years (Wilson et al., 1973), with some of the basic concepts dating from Bagnold""s (1956) work on the flow of cohesionless grains in fluids. For sliding abrasion, the erosion rate depends on the properties of particles and wear surfaces, the normal stress and the relative velocity.
Normal stress is enhanced when the flow streamlines are curved, as in an elbow. In this case, there is a centrifugal acceleration equal to u2/r, where u is the local velocity and r is the radius of curvature of the stream lines. This acceleration is often greater than that of gravity, producing a commensurate increase in the normal stress between the moving contact-load solids and the wall material, thus increasing the rate of sliding-abrasion wear. Such a wear type of behavior can cause elbows to wear, and is also very important in pump casings and along the surface impeller vanes where sliding abrasion in most areas tends to dominate.
A second type of wear is the particle-impact mode, which occurs where individual particles strike the wearing surface at an angle, despite the fact that the fluid component of the slurry is moving along the surface. Removal of material over time occurs through small-scale deformation, cutting, fatigue cracking or a combination of these, and thus depends on the properties of both the wearing surface and the particles. Ductile materials tend to exhibit erosion primarily by deformation and cutting, with the specific type depending on the angularity of the eroding particles. Brittle or hardened materials tend to exhibit fatigue-cracking erosion under repeated particle impacts. For a given slurry, the erosion rate depends on properties of the wearing surface such as hardness, ductility, toughness and microstructure. The mean impact velocity and mean angle of impact of the solids are also important variables, as are particle characteristics such as size, shape and hardness, and the concentration of solids near the surface.
The particle-impact type of erosion occurs because the trajectories of the individual particles do not follow the stream lines of the average flow. This behavior is important where the acceleration produced by strongly curving stream lines throws the particles towards a nearby surface, as occurs at the impeller vane inlets. Moreover, pumps and pipelines transporting water-based settling slurries almost invariably operate in highly turbulent eddies. The impact erosion associated with turbulence is best exemplified by the conditions in the upper portion of a settling slurry pipeline. Here removal of material occurs as in other types of impact erosion, although the impact vehicles and angles are more random.
It is expected that erosion by particle impact will be more effective than sliding abrasion provided that an equal number of particles are involved in each mechanism. The required conditions apply for low solids concentrations or cases where only a small fraction of the solids moves as contact load. The moving contact-load particles coming from upstream will be spaced sufficiently far apart to allow speedy incoming particles to erode the surface by impact. As a result, the local wear rate may be high. However, with larger solids concentrations the contact-load particles will be closer together.
An increase in particle concentration also results in increased wear. When this occurs as a result of a variation of concentration or stratification, then local wear will occur which could cause a drop-off in performance before the remainder of the pump part has achieved its full life.
It is also very common to see high local wear at the inlet edge of the vanes of an impeller when the slurry particle size is over 100 micron in size and the particles are concentrated towards the bottom on the impeller inlet.
Thus, there is a need for presenting a more uniform particle concentration profile to the centrifugal pump closer to that of zero velocity impact to reduce wear.
The present invention comprises a slurry conduit for connecting to a centrifugal pump for transporting a slurry mix of solids and liquids. The conduit includes an inclined upstream section having a reduced cross section whereby a concentration gradient formed within the slurry conduit is reduced. The inclined section forces any sliding bed or stratified slurry upwards as it goes towards the apex of the reduced section. As the slurry leaves the reduced cross section, it falls slowly back to the bottom of the pipe. Before the slurry can settle it enters the centrifugal pump. When entering the pump, the slurry has a substantially monolithic or nonstriated composition, which reduces the wear on the pump.
In an additional embodiment, the slurry conduit for connecting to a centrifugal pump for transporting a slurry mix of solids and liquids comprises a venturi section. The venturi section has a reduced cross section whereby a concentration gradient formed within the slurry conduit is reduced.
Furthermore, a process is provided for reducing a concentration and velocity gradient formed within a slurry conduit. The process comprises the steps of uplifting a portion of the slurry within the conduit and then delivering a substantially nonstriated slurry stream to a centrifugal pump.